Lawn mower

ABSTRACT

A lawn mower having a housing and a cutter unit positioned below the housing. The cutter unit comprises an upper rotating cutting unit and a lower stationary cutting unit that is concentric with the upper cutting unit and which have a plurality of upper and lower blades at their borders. Each blade has a cutting surface with a near end close to the border, and a far end away from the border. The blades of the upper cutting unit extend in front of a line passing through the concentric axis and the near end, in the direction of rotation of the upper cutting unit. 
     The upper blades rotate and slide over stationary lower blades, thereby cutting the lawn in a manner similar to a pair of scissors. The angle between the upper and lower blades is preferably 16-20 degrees. 
     The diameter of the upper cutting unit is greater than or equal to the diameter of the lower cutting unit, and the distance from the axis to the far end of the upper blades is greater than or equal to the distance from the axis to the far end of the lower blades. In addition, the lower blades, transverse to the direction of travel of the mower are more closely spaced and more numerous than in the direction of travel. 
     The lower cutting unit is prevented from rotating by a supporting arm, attached to the bottom of the housing, and having projections which engage openings in the lower cutting unit. The supporting arm and lower cutting unit can be rotated 90 degrees to accommodate a change in the direction of travel of the mower. 
     In another embodiment, a plurality of cutter units, all in the same plane, can be provided, along with a supporting plate for preventing the lower cutting units from rotating.

This is a continuation of application Ser. No. 454,328, filed Dec. 29,1982, now U.S. Pat. No. 4,520,619.

FIELD OF THE INVENTION

This invention relates to a lawn mower having a cutter unit includingupper and lower concentric cutting units which are each provided with aplurality of blades.

BACKGROUND OF THE INVENTION

Conventional lawn mowers having upper and lower cutting units aremanufactured so that the blades of the upper and/or lower cutting unitsare in the shape of saw-teeth or comb-teeth which project away from theaxis around which the units rotate. Because of their shape the blades donot uniformly and smoothly cut the lawn at right angles to the directionin which the mower travels. As a consequence, the mowing area is smallcompared to the size of the upper or lower cutting units. Furthermore,the lawn is mown unevenly and, therefore, the mowed area is irregularand not uniform. In addition, such a mower does not cut the lawnsharply. Finally, the lawn that is taken in between the two blades isapt to slip out of the space between the two blades and, therefore, themowing efficiency is low.

In order to overcome the drawbacks mentioned above, these lawn mowershave been redesigned so that both blades slide in parallel, on top ofeach other. Such a mower, however, has additional disadvantages. Forexample, it is difficult to position the blades so that they slide closeto each other. Furthermore, the blades are apt to bite each other andtherefore they are often damaged. In addition, because the load appliedto the mower fluctuates, depending on the amount of the law cut by theblades, a large prime mover or motor is necessary to power the mower.

The conventional mower of the type mentioned above, has a vertical shaftwhich supports the upper and lower cutting units. Thus, the cuttingunits are suspended from the shaft and therefore, they are apt to rock.When rocking of the blades occurs, the height of the lawn that is mowedbecomes uneven. Furthermore, when two or more of such cutting units arearranged in the same plane, there is often a space on the lawn which isleft unmowed between the units, or the cutting units may come intocontact with each other and break. Furthermore, when a supporting arm isprovided to prevent rocking of the cutter units, the screws forattaching the cutter units to the supporting arm are apt to become loosedue to the sliding movement of the cutter units when they mow the lawn.As a consequence, the cutter units rotate and become defective. It istherefore an object of the present invention to provide a lawn mowerhaving blades that cut the lawn sharply and do not rock, that smoothlycut grass at right angles to the mowing direction, and that cut the lawnuniformly without the blades becoming damaged.

SUMMARY OF THE INVENTION

According to the invention, the lawn mower comprises a housing and acutter unit positioned below the housing. The cutter unit includes alower cutting unit having at least one lower blade thereon, and an uppercutting unit, coaxial with the lower cutting unit around an axis andadapted to rotate in a direction t. The upper cutting unit includes atleast one upper blade thereon, and a cutting surface on the blade havinga near end. The upper blade extends in front of a line passing throughthe axis and the near end, in the direction t. The upper and lowercutting units may further include a border. The blades are attached atthe border to the upper and lower cutting units. The lawn mower mayfurther include a prime mover, in the housing, for rotating the lowerblade, and the upper and lower cutter units may further include aplurality of blades. The crossing angle between the upper blade and thelower blade is between approximately 16-20 degrees. In addition, themower may comprise a plurality of cutter units.

The upper and lower blades have a cutting surface with a near end, and afar end which is further away from the point at which the blades areattached to their respective borders than the near end. The distancefrom the axis to the far end of the upper blade is substantially equalto or greater than the distance from the axis to the far end of thelower blade. In addition, the distance from the axis to the border ofthe upper cutting unit is substantially equal to or greater than thedistance from the axis to the border of the lower cutting unit.Furthermore, the blade density of the lower cutting unit in thedirection transverse to the movement of the mower is sufficientlygreater than the blade density of the lower cutting unit in thedirection of movement of the mower, such that a lawn of uniform heightover the entire area covered by the cutting unit is produced.

In another embodiment, the lawn mower includes a housing and a cutterunit. The cutter unit includes an upper cutting unit below the housing,and a lower cutting unit below the housing, coaxial with the uppercutting unit, and adapted to cooperate with the upper cutting unit tocut a lawn. A supporting arm is attached to the housing and is adaptedto prevent the lower cutting unit from rotating. A fixed shaft isattached to the lower cutting unit, and the supporting arm is attachedto the fixed shaft and the bottom of the housing. A prime mover is alsoincluded in the housing, for rotating the upper cutting unit.

The lower cutting unit contains at least one opening therein, and thesupporting arm includes at least one projection thereon for engaging theopening in the lower cutting unit. The projection has an opening thereinwhich is adapted to allow grass which has become positioned between theupper and lower cutting units to fall therethrough. The supporting armmay be U-shaped in cross-section, with the leg of each "U" attached tothe bottom of the housing, or the supporting arm may be L shaped incross-section. A stop is attached to the housing, which abuts the legsof the U-shaped supporting arm, and is adapted to prevent the legs fromrotating. The supporting arm and the lower cutting unit are adapted torotate through 90 degrees, and then become rigidly attached to the fixedshaft.

The lower cutting unit has a border and at least one lower bladeattached to the border. The upper cutting unit is coaxial with the lowercutting unit around an axis and is adapted to rotate in direction t. Theupper cutting unit includes a border, at least one upper blade attachedto the border, and a cutting surface on the blade having a near end. Theupper blade extends in front of a line passing through the axis and thenear end, in the direction t. The crossing angle between the upper andlower blades is between approximately 16-20 degrees.

The upper and lower blades have a cutting surface with a near end, and afar end which is further away from the point at which the blades areattached to the borders than the near end. The distance from the axis tothe far end of the cutting surface of the upper blade is substantiallyequal to or greater than the distance from the axis to the far end ofthe cutting surface of the lower blade. The distance from the axis tothe border of the upper cutting unit is substantially equal to orgreater than the distance from the axis to the border of the lowercutting unit. In addition, the upper and lower cutting units maycomprise a plurality of upper and lower blades. Also, the blade densityof the lower cutting unit in the direction transverse to movement of themower is sufficiently greater than the blade density of the lowercutting unit in the direction of movement of the mower such that a lawnof uniform height over the entire area covered by the cutter unit isproduced.

In another embodiment, the lawn mower comprises a plurality of cutterunits in the same plane. Each of the cutter units comprises an uppercutting unit and a lower cutting unit coaxial with the upper cuttingunit to cut the lawn. In addition, the mower includes a supportingplate, attached to each of the lower cutting units, and adapted toprevent the lower cutting units from rotating. A plurality of fixedshafts is also provided so that one shaft is attached to each of thelower cutting units and the supporting plate. The lower cutting unitsmay further include at least one opening therein, and the supportingplate may include at least one projection thereon for engaging theopening, and adapted to prevent the lower cutting units from rotating.Each cutter unit has a center, and the supporting arms are positionedbehind a line passing through the centers of the cutter units, on theside of the line away from the predetermined direction in which themower moves.

In this embodiment, the lawn mower may also include a housing, with thecutter unit positioned below the housing. The lower cutting unit mayhave a border, and at least one blade attached to the border. The uppercutting unit, which is coaxial with the lower cutting unit around anaxis, is adapted to rotate in a direction t, and also includes a border,at least one upper blade attached to the border, and a cutting surfaceon the blade having a near end. The upper blade extends in front of aline passing through the axis and the first end portion in the directiont. The upper and lower cutting units may include a plurality of blades,and the crossing angle between each upper blade and lower blade isbetween approximately 16-20 degrees. The upper and lower blades have acutting surface with a near end, and a far end which is further awayfrom the point at which the blades are attached to their borders thanthe near end. The distance from the axis to the far end of the cuttingsurface of the upper blade is substantially equal to or greater than thedistance from the axis to the far end of the cutting surface of thelower blade. The distance from the axis to the border of the uppercutting unit is substantially equal to or greater than the distance fromthe axis to the border of the lower cutting unit. The blade density ofthe lower cutting unit in the direction transverse to the movement ofthe mower is sufficiently greater than the blade density of the lowercutting unit in the direction of movement of the mower such that a lawnof uniform height over the entire area covered by the cutter unit isproduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are disclosed in the accompanyingdrawings wherein:

FIG. 1 is a top plan view, partially in section, of an embodiment of acutter unit used in the lawn mower of the invention;

FIG. 2 is a top plan view of another embodiment of a cutter unit;

FIG. 3 is a top plan view of still another embodiment of a cutter unit;

FIG. 4 is a side sectional view of the cutter unit along line IV-IV ofFIG. 3;

FIG. 5 is a bottom view, partially in section, of an embodiment of thelawn mower according to the invention;

FIG. 6 is a side view, partially in section, of the embodiment shown inFIG. 5;

FIG. 7 is a bottom view of the embodiment of the mower showing asupporting arm that has been rotated 90 degrees together with a lowerblade;

FIG. 8 is a perspective view of an embodiment of a mower which mowsweeds according to the invention;

FIG. 9 is a sectional side view showing the mower shown in FIG. 8;

FIG. 10 is a bottom view, partially cut-away of an embodiment of thelawn mower of the invention;

FIG. 11 is a sectional side view of the mower along the line XI--XI ofFIG. 10;

FIG. 12 is a bottom view of another embodiment of the lawn mower of theinvention; and

FIG. 13 is a side view, partially cut-away, of the mower shown in FIG.12.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIGS. 1-4 illustrate various embodiments of the cutter unit A of thelawn mower. Each embodiment of cutter A comprises a top cutting unit 3and a bottom cutting unit 4. Units 3 and 4 are coaxial around a axis 0.Both top cutting unit 3 and bottom cutting unit 4 are circular and,therefore, contain circular borders 3' and 4' respectively. Attached atborders 3' and 4' are a plurality of projections or blades 3a and 4a,respectively, which contain the cutting surfaces 3b 4b and an inneredge. As illustrated in FIG. 1, the inner edge of cutter unit 3 hascutting surface 3b thereon, and the outer edge of cutter unit 4 hascutting surface 4b thereon. Cutting surface 3b starts at near end 3a'which is the point at which radial straight line 1 first intersectsblade 3a, and extends in a straight line to far end point 3a ", which isfurther from the point at which the blade is attached to the border,than near end 3a'. Cutting surface 4b is curved and has the shape of agently curving circle, extending in the direction of t in which theblades rotate. Alternatively cutting surface 3b can be in the shape ofgently curving circle extending in the direction t of the rotatingblades.

Cutting units 3 and 4 are arranged coaxially so that blades 3a and 4aslide on one another. When the two cutter units 3 and 4 are rotated inopposite directions, or when either cutter unit is rotated in directiont shown in FIG. 1 around axis 0, blades 3a and 4a snip blades of grassin a similar fashion to a scissors, thereby mowing the lawn.

The shapes and orientation of blades 3a and 4a are picked so that thecrossing angle θ between blades 3a and 4a is approximately 16-20degrees. Because the crossing angle is 16-20 degrees, blades 3a and 4acut the grass sharply, and the grass introduced between blades 3a and 4adoes not slip out of the space between blades 3a and 4a before beingcut.

In one embodiment, the shape and position of blade 3a is picked firstand therefore the shape and position of blade 4a is determined by blade3a, so that the crossing angle θ is 16-20 degrees. In the embodimentshown in FIG. 1 the crossing angle is approximately 16-20 degrees andthe inner edge of blade 3a has a cutting surface 3b that is linear andthe outer edge of blade 4a and cutting surface 4b are in the shape of agently sloping circle. Alternatively, blade 3a could have a cuttingsurface which is in the shape of a gently sloping circle and blade 4acould have a cutting surface in the shape of straight line. Or, bothblades 3a and 4a could have cutting surfaces in the shape of gentlysloping circle.

For safety reasons, it is preferable to rotate upper cutting unit 3 indirection t seen in FIG. 1, and to keep cutting unit 4 stationary,because a stationary lower cutting unit is less likely to injure theuser. It is also preferable to rotate only the top cutting unit becauseit is difficult to design the structure of the mower to rotate bothunits in opposite directions.

In order to insure that no grass strays into the space between upper andlower cutting units 3 and 4, thereby dulling the blades, and in order toinsure that blades 3a and 3b do not bite each other, the diameter 1₃ ofupper cutter unit 3, (that is, the distance from O to border 3'), ispicked to be substantially equal to or greater than 1₄, the diameter oflower cutter unit 4 (that is, the distance from O to border 4'), so that1₃ ≧1₄. In order to insure that the grass will always be taken inbetween blades 3a and 4a, the diameter of upper cutter unit 3 is chosenso that the length 1₁ (the distance between O axis and the far end 3a"of blade cutting surface 3b) is substantially equal to or greater than1₂ (the distance between axis O and the far end 4a" of cutting surface4b), so that 1₁ 1₂.

As seen in FIG. 4, cutting surface 3b of upper cutting unit 3 is turnedslightly upward so that cutting surface 3b slides close to cuttingsurface 4b of lower cutting unit 4.

Because the direction in which the mower moves is predetermined (forexample, in the direction m in FIG. 3), there is no need to have bladesof the same shape in order to cut the grass to a uniform height, as longas the crossing angle is between 16-20 degrees, and, one of the twocutting units does not rotate. For example, the embodiments illustratedin FIGS. 3 and 4 show a lower cutter unit 4 that is fixed, and an uppercutting unit that is rotatable. Furthermore, lower cutter unit 4 isprovided with dense teeth (i.e., closel spaced teeth) on the right andleft sides thereof, and thin teeth (i.e., widely spaced teeth) on thefront and rear portions thereof. If the lawn mower is moved in the frontdirection (e.g., direction m in FIG. 3), the grass directly in front ofthe mower will be cut by blades at the front and the rear of the mower.Grass that is not directly in front of the mower will only be cut by oneset of blades -- those on the right or left side of the mower (m₁, inFIG. 5, m₂ in FIG. 7). In order to cut the same amount of grass so as toproduce a lawn of uniform height, the lower blades in the directiontransverse to the movement of the mower are more numerous and moreclosely spaced than lower blades positioned in the direction of movementof the mower; thus grass is introduced smoothly into the mower, and thesame quantity of grass is cut in front of and on each side of the mowerso as to produce a lawn of uniform height.

Thus, cutter unit A comprises an upper cutting unit 3 and a lowercutting unit 4. Blade 3a of the upper cutting unit 3 slides on blade 4aof lower cutting unit 4 from points 3a', 4a' to 3a", 4a", respectively,and as blade 3a rotates, blade 3a pushes grass toward blade 4a so as tocut the grass in light, quick strokes. One, or a plurality of cutterunits A may be arranged in the same plane below housing 2. FIGS. 5through 13 illustrate alternative embodiments of the mower. FIGS. 5through 7 show a hand-operated lawn mower. FIGS. 8 and 9 show a weedmower. FIGS. 10 and 11 illustrate a mower having two cutter units A1,A2, and, FIGS. 12 and 13 show a mower provided with three cutter unitsA1, A2 and A3.

The hand-operated mower shown in FIGS. 5 through 7, includes abox-shaped housing 2; housing 2 in turn houses a prime mover 1, a fan 8for cooling prime mover 1, and a driving mechanism 9 for driving thecutter unit A. Below housing 2 are provided a cutter unit A having upperand lower cutting units 3 and 4 respectively, and a supporting arm 6 forpreventing lower unit 4 from being rotated.

Upper cutting unit 3 is attached to a rotary shaft 10 providedvertically below housing 2, while lower cutting unit 4 is fixedlyattached to a fixed shaft 5 by a bolt 11. By connecting rotary shaft 10to driving mechanism 9 of the prime mover 1, and by rotating uppercutting unit 3 in the direction shown by arrow t, the lawn is mownquickly and lightly by blades 3a and 4a.

As seen in FIGS. 5 through 7, the mower also includes an air inlet 12,an air outlet 13, a grip 14 formed integrally with the housing 2, anoperating switch 15, and a safety stopper 16 for preventing switch 15from being inadvertently activated.

Supporting arm 6, which prevents lower cutting unit 4 from beingrotated, extends between the two sides of the bottom of housing 2 as isclear from FIG. 7. Arm 6 is connected to lower cutting unit 4 and isrigidly attached to the fixed shaft 5. Arm 6 comprises a circular center6a, which is slightly smaller than the lower blades 4a; connections 6b,6b positioned between the two sides of housing 2, on either side ofcenter 6a; and legs 6c, 6c extending vertically from each end ofconnections 6b, 6b, so that supporting arm 6 is in the shape of a U incross-section.

Center 6a is provided with a plurality of ring projections 18 whichengaged openings 17 in lower cutting unit 4. Center 6a is rigidlyattached, together with lower cutting unit 4, to the bottom of fixedshaft 5. In addition, ring projections 18 engage openings 17 in thelower cutting unit 4 so that the rotation of lower cutting unit 4 may befirmly prevented.

Openings 17 function to engage projections 18, and to decrease theweight of lower cutting unit 4. Furthermore, they function as outletsfor falling grass that has inadvertently become positioned between thetwo cutting units 3 and 4. For this purpose, ring projections 18 ofsupporting arm 6 have an opening therein and are hollow so as to formoutlets 18a for the grass trapped between cutter units 3 and 4.

As shown in FIG. 6, the bottom of housing 2 is provided with projections19 which abut legs 6c, 6c so that legs 6c, 6c may not rotate. When arm 6is rigidly attached to fixed shaft 5 by means of a bolt 11, legs 6c, 6care fixed to the bottom of housing 2 and abut projections 19, so thatthe arm 6 cannot be rotated. When bolt 11 is loosened, arm 6 and lowercutter unit 4 can be rotated 90 degrees so that they may be oriented inthe direction in which the mower travels, i.e., the direction shown byarrow m1 in FIG. 4 or by m2 in FIG. 7).

Alternatively, supporting arm 6 may be L shaped (not shown). In thisembodiment, its leg is fixed to the bottom of the housing 2 while itscenter, and lower cutter unit 4 are attached to fixed shaft 5.

FIGS. 8 and 9 illustrate an embodiment of a mower which cuts weeds. Itcomprises an operating arm 20, provided at one end with a prime mover 1,and at the other end with a housing 2. A cutter unit A comprising upperand lower cutting units 3 and 4 is provided below housing 2.

According to this embodiment, upper cutting unit 3 is fixed to avertical rotary shaft 10, provided within the housing 2, while lowercutting unit 4 is attached to a fixed shaft 5 which is verticallyaligned with and coaxial with rotatable shaft 10. Operating arm 20 isprovided therein with a transmitting mechanism 21 which transmits powergenerated by prime mover 1 to rotary shaft 10.

When transmitting mechanism 21 is connected to a driving mechanismhoused in the housing 2, upper cutting unit 3 is rotated. Consequently,a blade 3a of rotary cutting unit 3 in combination with a blade 4a ofstationary lower cutting unit 4 mows the weeds.

FIGS. 10 through 13 show hand-powered or electric powered lawn mowers.The mower is provided with a plurality of cutter units A1, A2, A3 in thesame plane, and with a supporting plate 7 provided below lower cuttingunits 4.

Each of the upper cutting units 3 of cutter units A1, A2, A3 is attachedto a rotary shaft 10 extending vertically to the bottom portion of ahousing 2. Each lower cutting unit 4 is attached to the bottom of afixed shaft 5 which is coaxial with rotary shaft 10. When rotary shafts10 of respective cutter units A1, A2 and A3 are rotated in synchronismwith each other by means of gears 23a, 23b, 23c connected to a primemover 1, upper cutting units 3 are rotated in a direction shown by anarrow t. In this way, upper blades 3a in combination with lower blades4a, snip blades of grass, thereby mowing the law.

Cutter units A1, A2 and A3 are positioned at the same height, separatedby a predetermined distance. A support plate 7 is provided below lowercutting units 4. Support plate 7 is thin and connected together withlower cutter units 4, to respective fixed shafts 5 by bolts 11. Plate 7is provided with projections or stopping pins 25 which engaged openings24 in lower cutting units 4 so that cutting units 4 cannot rotate. Plate7 is positioned behind a line which passes through the centers ofrespective cutter units A1, A2, and A3 on the side of the line away fromthe direction the mower moves, so that the lawn is not prevented fromentering blades 3a and 4a by support plate 7.

To summarize, the lawn mower, according to the present invention,includes at least one cutter unit A comprising an upper cutting unit 3provided at its circumference with a plurality of blades and a coaxiallower cutting unit 4. The blade 3a of upper cutting unit 3 is positionedin front of a line 1 in FIG. 1 (that is, on the side of line 1 in thedirection in which blade 3a rotates) which starts at axis O andintersects blade 3a at near end 3a '. Blade 3a slides over blade 4a oflower cutting unit 4, from their near ends 3a ', 4a' to their far ends3a ", 4a". Upper blade 3a moves in direction t, thereby pushing thegrass against lower blade 4a and snipping the grass and mowing the lawn.

The lawn mower of the present invention contains a number of advantages.First, because the action of blades 3a and 4a is similar to a pair ofscissors, the grass is continuously snipped, thereby producing a lawnthat is beautiful to look at. Second, because the amount of lawn that iscut over time is uniform, the load applied to the mower does notfluctuate very much during its operation and therefore the powerrequired by the motor can be reduced. Consequently, the mower can bemade small and inexpensively, and therefore one can use the presentinvention for long periods of time without becoming tired.

Third, the upper blade can take in and cut grass at right angles to thedirection in which the mower is traveling, thereby greatly improvingmowing efficiency. Fourth, because it is not necessary to rotate theupper blade (or the lower blade) extremely fast, pebbles or twigs thatstray into the mower will not be flipped away by the blades at highspeeds. Fifth, the upper and lower blades can be easily andinexpensively produced by die-cast molding. Furthermore, theirreplacement and grinding can be easily effected. Sixth, because thecrossing angle between the two blades is maintained constantly between16-20 degrees, the blades sharply cut the grass, and the lawn that isintroduced between the blades will not slip out.

Seventh, because the distance from axis O to the far end 3a" of blade 3ais substantially greater than or equal to the distance from axis O tothe far end 4a " of blade 4a, the grass over which the mower moves isalways taken in between blades 3a and 4a. Eighth, the diameter of theupper cutting unit 3 (i.e., the distance between O and border 3') issubstantially equal to or greater than the diameter of lower cuttingunit 4 (i.e., the distance from O to circular border 4'). Therefore, thegrass cannot fall into the space between the upper and lower cuttingunits 3 and 4 which would produce dull blades. In addition, this insuresthat the two blades do not bite each other. Nineth, two or more cutterunits can be arranged in the same plane, thereby allowing a broad areato be mowed, even though the frame or housing of the mower is not large.

The lawn mower of the present invention also includes a supporting arm 6which prevents lower blade 4a from rotating and which forms the bottomof the housing. Supporting arm 6 gives the present invention a number ofadditional advantages. The lower blade is much less likely to break downdue to long use and, therefore, maintenance and inspection are notnecessary for long periods of time. In addition, the supporting armfunctions as a guide sledge when the mower moves over the lawn. This isespecially important in a handoperated mower, which can consequently beoperated with ease. Furthermore, the supporting arm protects theoperator from being injured by the cutter unit. Finally, becausesupporting arm 6 together with the lower blade 4a can be rotated by 90degrees, a uniform, smooth and beautiful mown lawn is obtained, even ifthe direction of the mower is changed.

The lawn mower may also include a supporting plate which functions as abridge between a plurality of lower cutting units arranged in the sameplane.

Because each of the lower cutting units is prevented from rotating bythe supporting plate, the lower cutting units will not rotate andthereby break down even if the mower is used over a long period of time.In addition, the supporting plate allows a plurality of cutter units tobe supported in a stable fashion at predetermined positions so that theywill not rock during operation. Furthermore, because the cutter unitsare arranged close to each other, the lawn will not be mown unevely orirregularly.

The present invention has been so far described with particularreference to several illustrative embodiments thereof, but it will beunderstood that variations and modifications can be effected withoutdeparting from the true spirit of the present invention as describedhereinabove and as defined in the appended claims.

What is claimed is:
 1. A lawn mower comprising: a cutter unitincluding:(a) a lower cutting unit having at least one lower bladehaving a cutting surface, said blade being attached to said lowercutting unit; and (b) an upper cutting unit coaxial with said lowercutting unit around an axis, wherein said upper cutting unit is adaptedto rotate in a direction t around said axis and wherein said uppercutting unit comprises: at least one upper blade thereon, and a cuttingsurface on said upper blade, having a first end and an second end,wherein said first end is closer to said axis than said second end,wherein said upper blade cutting surface extends in front of a linepassing through said axis and said first end, in said direction t,wherein the crossing angle between said upper blade cutting surface andsaid lower blade cutting surface is between approximately 16 and 20degrees, and means for maintaining said angle between said blades withina range between 16 and 20 degrees during rotation of one of said cuttingunits, wherein one of said blades has a linear cutting surface andwherein the other of said blades has an arcuate cutting surface.
 2. Thelawn mower of claim 1 further including a housing, wherein said cutterunit is positioned below said housing.
 3. The lawn mower of claim 1wherein said upper and lower cutting units include a border and whereinsaid blades are attached at said border to said upper and lower cuttingunits.
 4. The lawn mower of claim 1 wherein said upper and lower cuttingunits further includes a plurality of blades.
 5. The lawn mower of claim2 further including a prime mover, in said housing for rotating saidupper blade.
 6. The lawn mower of claim 3 wherein said upper and lowerblades have a cutting surface with a near end, and a far end that isfurther away from the point at which said blades are attached to theirrespective borders, then said near end, wherein the distance from saidaxis to said far end of said upper blade is substantially equal to orgreater than the distance from said axis to said far end of said lowerblade.
 7. The lawn mower of claim 3 wherein the distance from said axisto said border of said upper cutting unit is substantially equal to orgreater than the distance from said axis to said border of said lowercutting unit.
 8. The lawn mower of claim 1 comprising a plurality ofcutter units.
 9. The lawn mower of claim 4 wherein the blade density ofsaid lower cutting unit in the direction transverse to the movement ofsaid mower is sufficiently greater than the blade density of said lowercutting unit in the direction of movement of said mower such that a lawnof uniform height over the entire area covered by said cutter unit isproduced, wherein said blade density of said lower cutting unit isdefined as the number of blades in a particular direction over a unitlength in that direction.
 10. A lawn mower in accordance with claim 1wherein said lower blade cutting surface is circular and said upperblade cutting surface is linear.
 11. A lawn mower in accordance withclaim 1 wherein said upper blade cutting surface is circular and saidlower blade cutting surface is linear.
 12. A lawn mower comprising:(a) aplurality of cutter units in the same plane, each of which comprises:(i)an upper cutting unit having a cutting surface; (ii) a lower cuttingunit having a cutting surface, wherein said lower cutting unit iscoaxial with said upper cutting unit around an axis and adapted torotate in a direction t around said axis and cooperate with said uppercutting unit to cut said lawn, wherein said cutting surface on saidlower cutting unit has a first end and a second end, wherein said firstend is closer to said axis than said second end, wherein said cuttingsurface of said lower cutting unit extends in front of a line passingthrough said axis and said first end, in said direction t, wherein thecrossing angle between said upper blade cutting surface and said lowerblade cutting surface is between approximately 16 and 20 degrees, andwherein one of the cutting surfaces of said lower cutting unit and saidupper cutting unit is circular, the other of said cutting surfaces beinglinear, said cutting surfaces together comprising means for maintainingsaid crossing angle between 16 and 20 degrees during rotation of one ofsaid cutting units.
 13. The lawn mower of claim 12 further including asupporting plate, attached to each of said lower cutting units andadapted to prevent said lower cutting units from rotating, wherein saidlawn mower further comprises a plurality of fixed shafts, one of saidfixed shafts being attached to each lower cutting unit and saidsupporting plate.
 14. The lawn mower of claim 12 wherein said mowerfurther comprise a supporting plate, attached to each of said lowercutting units and adapted to prevent the lower cutting units fromrotating, wherein said lower cutting units further include at least oneopening therein, and said supporting plates include at least oneprojection thereon for engaging said opening.
 15. The lawn mower ofclaim 12 wherein said mower moves in a predetermined direction, andwherein each cutter unit has a center and a supporting arm, attached tosaid housing and adapted to prevent said lower cutting unit fromrotating, and wherein said supporting arm is positioned behind a linepassing through the centers of said cutter unit, on the side of saidline away from the said direction in which said mower moves.
 16. Themower of claim 12 wherein said lower cutting unit has a border and atleast one lower blade attached to said border, and wherein said uppercutting unit is coaxial with said lower cutting unit around an axis, andis adapted to rotate in a direction t, and includes: a border, at leastone upper blade attached to said border, a cutting surface on saidblade, having a near end, wherein said upper blade extends in front of aline passing through said axis and said near end, in said direction t.17. The mower of claim 16 wherein said upper blades have a cuttingsurface with a near end and a far end further away from the point atwhich said blades are attached to their respective borders, than saidnear end, wherein the distance from said axis to said far end of saidcutting surface of said upper blade is substantially equal to or greaterthan the distance from said axis to said far end of said cutting surfaceof said lower blade.
 18. The mower of claim 16 wherein the distance fromsaid axis to said border of said upper cutting unit is substantiallyequal to or greater than the distance from said axis to said border ofsaid lower cutting unit.
 19. The lawn mower of claim 16 wherein theupper and lower cutting units comprise a plurality of upper and lowerblades, where in the blade density of said lower cutting unit in thedirection transverse to the movement of said mower is sufficientlygreater than the blade density of said lower cutting unit in thedirection of movement of said mower such that a lawn of uniform heightover the entire area covered by said cutter unit is produced.
 20. Thelawn mower of claim 12 further comprising a housing, wherein said cutterunit is positioned below said housing, and said upper cutting unit iscoaxial with said lower cutting unit around an axis and further includesa border and at least one upper blade attached to said border, and acutting surface on said blade, having a near end wherein said upperblade extends in front of a line passing through said axis and said nearend, in said direction t.
 21. The lawn mower of claim 20 wherein saidupper and lower cutting units further includes a plurality of blades.22. The lawn mower of claim 20 wherein said upper and lower blades havea cutting surface with a near end and a far end that is further awayfrom the point at which said blades are attached to said borders thansaid near end, wherein the distance from said axis to said far end ofsaid upper blade is substantially equal to or greater than the distancefrom said axis to said far end of said lower blade.
 23. The lawn mowerof claim 20 wherein the distance from said axis to said border of saidupper cutting unit is substantially equal to or greater than thedistance from said axis to said border of said lower cutting unit. 24.The lawn mower of claim 24 wherein the blade density of said lowercutting unit in the direction transverse to the movement of said moweris sufficiently greater than the blade density of said lower cuttingunit in the direction of movement of said mower such that a lawn ofuniform height over the entire area covered by said cutter unit isproduced.
 25. A lawn mower in accordance with claim 12 wherein saidlower blade cutting surface is circular, and said upper blade cuttingsurface is linear.
 26. A lawn mower in accordance with claim 12 whereinsaid upper blade cutting surface is circular, and said lower bladecutting surface is linear.
 27. A weed cutter comprising(a) an uppercutting unit; (b) a lower cutting unit, below said upper cutting unit,wherein said upper and lower cutting units each comprise a blade,wherein the crossing angle between said blade cutting surfaces isapproximately 16 and 20 degrees, wherein one of said blades has anarcuate cutting surface and the other of said blades has a linearcutting surface, wherein the shape of said blade cutting surfacescomprise means for maintaining said crossing angle at between 16 and 20degrees during rotation of one of said cutting units; (c) a motor fordriving said upper cutting unit and comprising means for being carriedby a user; (d) means for supporting said upper and lower cutting units,wherein said upper and lower cutting units extend beyond said support ina horizontal plane; (e) means for operatively connecting said motor tosaid upper cutting unit; (f) a rotating shaft, attached to said supportmeans and said connecting means, for rotating said upper cutting unit;and (g) a fixed shaft, coaxial with and vertically aligned with saidrotating shaft, and connecting said lower cutting unit to said supportmeans for maintaining said lower cutting unit in a stationary positionwith respect to said support means.
 28. The cutter defined by claim 27,wherein said rotating shaft has a substantially vertical openingtherein, and wherein said fixed shaft is disposed in said opening. 29.The cutter defined by claim 28, wherein said upper and lower cuttingunits comprise upper and lower blades, respectively, which extend beyondsaid support means, in a horizontal plane.
 30. The cutter defined byclaim 24 wherein said upper cutting unit is coaxial with said lowercutting unit around an axis, wherein said upper cutting unit is adaptedto rotate in a direction t around said axis and wherein said uppercutting unit comprises a cutting surface on said blade, having a firstend and a second end, wherein said first end is closer to said axis thansaid second end, wherein said cutting surface extends in front of a linepassing through said axis and said first end, in said direction t.
 31. Alawn mower in accordance with claim 27 wherein said lower blade cuttingsurface is circular and said upper blade cutting surface is linear. 32.A lawn mower in accordance with claim 27 wherein said upper bladecutting surface is circular and said lower blade cutting surface islinear.